Several lines of evidence suggest that accumulation of DNA damage coupled with defects in DNA repair play an important role in breast cancer susceptibility and perhaps in tumor progression. This evidence includes data showing that levels of oxidative DNA damage are increased in human breast tumors and in surrounding normal tissue. It has also been shown that the breast cancer susceptibility gene, BRCA1, is required for transcription coupled repair of oxidative damage in mutant rodent models. Our own previous work has shown that nucleotide excision repair is defective in the Li-Fraumeni syndrome, a heritable cancer prone syndrome associated with increased susceptibility to breast cancer. Finally, several groups have shown that defects in the removal of UV- and X-irradiation induced DNA damage are present in newly diagnosed sporadic breast cancer patients and in their healthy first- degree female relatives. These data suggest that both the nucleotide excision and base excision repair pathways may be involved in breast cancer development. It remains to be established whether DNA repair mechanisms are defective in breast cancer cells and which specific repair pathways are of primary importance. We hypothesize that one of the critical steps in mammary carcinogenesis is the loss of the normal response to DNA damage and that specific defects in nucleotide excision repair may be critical in development and progression of the malignant phenotype. Acquired defects in nucleotide excision DNA repair during the process of tumorigenesis may involve a number of genes including well known DNA repair related genes as well as breast cancer susceptibility genes BRCA1 and BRCA2.In the present study, we examine transcription coupled nucleotide excision repair as well as ultraviolet- light induced overall repair capacity in normal human mammary epithelial cells (HMEC) AG11134 and the breast cancer cell lines MCF7 and MDA-MB-468. Examination of overall genome repair by unscheduled DNA synthesis reveals that both MCF-7 and MDA-MB-468 cells have defective repair of UV-induced lesions in the overall genome. In addition, the two breast cancer cell lines exhibit reduced RNA synthesis recovery suggesting a possible defect in repair of actively transcribing sequences. Transcription coupled repair of cyclobutane pyrimidine dimers examined using the gene specific repair assay (Bohr assay) is reduced in both tumor cell lines when compared to normal human mammary epithelial cells and normal skin fibroblasts. These results suggest that these breast cancer cell lines are defective in nucleotide excision repair; however, the mechanism and genetic changes that account for this repair phenotype are unclear. We are currently examining the mRNA and protein expression levels of nucleotide excision repair related genes (ERCC1, XPA,, XPB, ,XPF, XPC and RPA) before and after UV irradiation. Preliminary results suggest an alteration in the expression of the ERCC1 and XPA gene at the protein level possible pinpointing defects in two steps pf the nucleotide excision repair pathway. Transcription coupled repair is also being assayed in other breast cancer tumor cell lines to discern whether other breast cancer cell lines are also repair deficient. On going work is examining mechanistic explanations focused on alterations in repair related gene function. - DNA damage, DNA repair, breast cancer, oxidative damage, hormones, transcription, chromatin